Sputtering, alternatively called physical vapor deposition (PVD), has long been used in depositing metals and related materials in the fabrication of semiconductor integrated circuits. Use of sputtering has been extended to depositing metal layers onto the sidewalls of high aspect-ratio holes such as vias or other vertical interconnect structures, as well as in the manufacture of extreme ultraviolet (EUV) mask blanks. In the manufacture of EUV mask blanks minimization of particle generation is desired, because particles negatively impact the properties of the final product.
Plasma sputtering may be accomplished using either DC sputtering or RF sputtering. Plasma sputtering typically includes a magnetron positioned at the back of the sputtering target including two magnets of opposing poles magnetically coupled at their back through a magnetic yoke to project a magnetic field into the processing space to increase the density of the plasma and enhance the sputtering rate from a front face of the target. Magnets used in the magnetron are typically closed loop for DC sputtering and open loop for RF sputtering.
In plasma enhanced substrate processing systems, such as physical vapor deposition (PVD) chambers, high power density PVD sputtering with high magnetic fields and high DC Power can produce high energy at a sputtering target, and cause a large rise in surface temperature of the sputtering target. The sputtering target is cooled by contacting a target backing plate with cooling fluid. In plasma sputtering as typically practiced commercially, a target of the material to be sputter deposited is sealed to a vacuum chamber containing the wafer to be coated. Argon is admitted to the chamber. When a negative DC bias of several hundred volts is applied to target while the chamber walls or shields remain grounded, the argon is excited into a plasma. The positively charged argon ions are attracted to the negatively biased target at high energy and sputter target atoms from it.
Erosion of the target causes a problem in plasma sputtering. As the target is eroded, the sputtering surface within the target layer recedes and comes closer to the magnetron so that the magnetic field at the sputtering surface changes over the lifetime of the target. The sputtering rate depends on the magnitude of the magnetic field adjacent the sputtering surface, which increases with the depth of erosion. Also, the plasma may become unstable under changes of magnetic field, possibly extinguishing or sparking, the latter of which can create damaging particulates. Regardless of the target shape, the target erodes more preferentially at specific locations with respect to the magnetic fields of the magnets of the magnetron causing an uneven or asymmetric erosion profile. The uneven erosion profile of the target may result in poor uniformity of the deposited film and uneven film characteristics across the substrate. For example, poor step coverage may be achieved at some spatial locations of the substrate while good step coverage may be achieved at other areas of the substrate. There is need to provide apparatus and methods to reduce asymmetry of erosion profiles and provide erosion profiles that are more even.